Single and double chip spare

ABSTRACT

Techniques are provided for overcoming failures in a memory. One portion of the memory may operate in a single chip spare mode. Upon detection of an error in a single chip in the portion of the memory, a region of the portion of the memory may be converted to operate in a double chip spare mode. The memory may be accessed in both single and double chip spare modes.

BACKGROUND

Computing devices may suffer from any number of different types offailures. One particular type of failure is failure in the memory chipsthat make up the memory system. Failures of individual memory chips mayreduce the amount of memory available to the system. As memory chips aregenerally not individually serviceable, replacement of a failed memorychip may require the complete replacement of a memory module that maycontain many non-failed chips. Furthermore, memory is typically notserviceable while the computer is operational and thus may requiredowntime to carry out the replacement of the failed memory modules.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a block diagram of an example of a memory system.

FIG. 2 depicts another block diagram of an example of a memory systemwith a chip failure.

FIGS. 3(a-b) depict additional examples of a memory system with chipfailures.

FIGS. 4(a-c) depict an example of a memory system with multiple chipfailures.

FIG. 5 depicts an example flow diagram of recovering from a chipfailure.

FIG. 6 depicts another example flow diagram of recovering from a chipfailure.

FIG. 7 depicts an example flow diagram of recovering from a chipfailure.

FIG. 8 depicts an example flow diagram of recovering from multiple chipfailures.

DETAILED DESCRIPTION

Memory failures in computing systems may lead to unplanned systemdowntime, due to the need to replace faulty memory components. Unplanneddowntime in today's computing environment, in which high availability ofcomputing systems is expected, may be intolerable. Problems due tomemory failure may be further exacerbated when the failures occur inserver systems, as many different users may be impacted by systemdowntime. In addition, many organizations rely on continuousavailability of applications running on their computers in order toachieve the organization's goals. Unplanned downtime due to memoryfailure can be detrimental to those goals.

To mitigate the effects of failure of memory, several techniques areavailable. One such technique may be referred to as Single Chip Spare(SCS). In SCS systems, a memory controller may store cache lines in thememory such that failure of any single memory chip does not result inthe cache line becoming unusable. For example, the cache lines may bestored in memory with the protection of an Error Correction Code (ECC).An ECC that is capable of detecting and correcting a failure of a singlechip may be chosen from a number of available error correction codes.Upon a failure of a chip in a memory, the ECC may be used to ensure thatthe cache lines are still usable.

An SCS system is able to continue running even in the presence of afailure of a single memory chip within an ECC codeword. Furthermore, SCSsystems are able to utilize the bandwidth between the memory and thememory controller in a highly efficient manner. However, SCS systemssuffer from the inherent problem that they can only recover from asingle chip failure within an ECC codeword. Failure of a second memorychip within the codeword may, at minimum, result in the need for systemdowntime to replace the faulty memory.

Another technique that may be used to mitigate failures in memory may bereferred to as Double Chip Spare (DCS). In DCS, cache lines are againstored in memory with the protection of one of many known errorcorrection codes that are capable of detecting and correcting for up totwo memory chip failures within each ECC codeword. Error correctioncodes that are capable of correcting faults in two error chips typicallydistribute a cache line across a larger number of chips, in comparisonto SCS, to reduce the number of bits that are corrupted when a chipfails. For example, a cache line may be distributed between chipscontained on two separate memory busses or between chips on a single busbut spread between addresses on a single bus such that any error of twochips storing the cache line does not make the line unusable. The cacheline distribution can occur between memory modules on different buses,different memory modules on the same bus, or different ranks on the samememory module.

DCS has the advantage of being able to survive failures in two memorychips within an ECC codeword before requiring downtime for repair. Thus,a first failure may occur, and the system may continue to operate. Evenwhen a second failure occurs, the system continues to operate. Thus, asystem administrator may be given a wider window of opportunity toaddress a memory failure during a planned maintenance period, ascompared to SCS. The failure of the first memory chip does not result ina situation where the next failure may cause the system to be unusable.Maintenance may not be needed until the failure of a second chip, whichmay not occur until a considerable amount of time after the first chipfailure.

However, the increased protection offered by DCS comes at a price. Asmentioned, DCS operates by storing cache lines across multiple busses ormultiple distinct ranges of memory addresses within a single bus. Theneed to access multiple busses or address ranges results ininefficiencies in the transfer of data from the memory to the memorycontroller, as extra cycles are needed to configure reading from/writingto different busses or different ranges on a single bus. As much as 40%of the bus bandwidth may be lost in order to gain the protection of DCS.The loss of bus bandwidth occurs even if there has not yet been afailure of any memory chip.

Techniques provided herein overcome the problems created by SCS and DCS,while at the same time retaining the advantages provided by those twomechanisms. A memory may initially operate in SCS mode, providing thethe benefits, such as full bus utilization, described above. When afailure in a single chip in memory occurs, the portion of the memoryaffected by the failure may be converted to DCS mode. Portions of memorythat are not affected remain operating in SCS mode. Thus, full bandwidthis maintained for those portions of memory that are not affected by thefailure of the memory chip, maintaining the advantages of SCS to theextent possible. Likewise, for the portion of the memory that contains afaulty memory chip, the conversion to DCS retains the advantage that thenext failure of a memory chip is not critical, as a failure of anadditional memory chip can be accommodated. The techniques above aredescribed in further detail below and in conjunction with the attachedfigures.

FIG. 1 depicts a block diagram of an example of a memory system. Thesystem may include a memory controller 110, memories 115-0, 115-1, andbusses 120-0, 120-1 which connect the memory controller to the memories.Memory controller may receive requests 125 to read/write cache lines inthe memory from external sources. For example, memory requests may comefrom sources such as processors, direct memory access controllers, orother such devices.

The memory controller may typically be implemented in hardware. Due tothe processing speed requirements needed in today's computingenvironment, the memory controller may generally be implemented as anapplication specific integrated circuit (ASIC) or may be included aspart of a larger device, such as a processor. The memory controller maycontain memory, general purpose processors, and dedicated control logic.In addition, the memory controller may execute software or firmware toaid in providing the functionality described herein. The variousfunctions that are described may be implemented using any combination ofthe memory, processors, software, firmware, and logic as needed.

The memory controller 110 may access the memory 115 over one or morebusses. Upon receiving a memory request, the memory controller mayaccess the memory in order to service the request. For example, in thecase of a request to read a cache line from memory, the memorycontroller may retrieve the cache line from the memory over the bus,compute the appropriate ECC based on the mode of operation, compare thecomputed ECC and ECC retrieved with the cache line, correct any errorsto the extent possible given the type of ECC, and provide the cache lineto the requestor. In the case of a request to write to memory, thememory controller may receive a request to write a cache line, computean appropriate ECC, and store the cache line in the memory.

In the description above, reference was made to an appropriate ECC. Aswas mentioned above, error correction codes are available for both SCSand DCS. Cache lines may be stored in ether SCS mode 130-0,1 or DCS mode135 in the memory. When reading or writing cache lines, the memorycontroller may select the proper ECC to use based on if the cache lineis contained in a SCS or DCS portion of the memory. Thus, the memorycontroller is able to access some portions of the memory in SCS mode andother portions of the memory in DCS mode. In addition, upon failure of achip in the SCS portion of memory, the memory controller may convert aregion of memory from SCS mode to DCS mode. Access of the memory in SCSand DCS mode, along with conversion from one mode to the other, isdescribed in further detail below.

FIG. 2 depicts another block diagram of an example of a memory systemwith a chip failure. In a typical memory system, cache lines containingsixty four bytes of data 205 may be used. A cache half line, as the nameimplies, is half of a complete cache line. For example, as shown thesixty four byte cache line 283 may be made up of half cache lines283-a,b. For purposes of the remainder of this disclosure, references toa cache line by number only indicate the full cache line, whilereferences to the number and a letter indicate reference to a singlehalf cache line.

The cache half lines may be stored in a memory 215-0,1. An example of atypical chip layout of a memory using eighteen ×4 Dynamic Random AccessMemory (DRAM) chips 208 is shown. In such an implementation, sixteen ofthe memory chips may be used to store data while two of the memory chipsare used to store an ECC. Given a total of eighteen chips, there aremany known single chip error correction codes that can detect andcorrect a failure in a single chip.

Cache lines 280-283 may be stored in memory 215-0. Cache lines 280-283may be protected by any available ECC that is able to detect and correcta single chip error. Cache lines 280-283 may form a first SCS region230-0. Likewise, cache lines 290-293 may be stored in memory 215-1.Cache lines 290-293 may also be protected by a single chip spare ECC andform a second SCS region 230-1. Each of these sets of cache lines isoperating in single chip spare mode, as failure of any single chip inthese regions does not render the cache lines stored therein unusable.The two SCS regions 230-0,1 are independent of each other.

Double chip spare region 235 is an example of a region that is protectedby an ECC that is capable of detecting and correcting failures in up totwo chips. This DCS region comprises cache lines within two addressranges: 240-241 and 250-251. As shown, each cache line is spread acrosstwo memory busses. For example, cache half lines (240,250,241,251)-a arestored in memory 215-0, while cache half lines (240,250,241,251)-b arestored in memory 215-1. Thus, a given cache line in a DCS region isstored across a total of thirty six chips, with thirty two chips usedfor data and four chips used to store an ECC. Given this number ofchips, error correction codes that are able to detect and correct forfailures of up to two chips are available. Cache lines 240-241 and250-251 are protected by such an ECC.

When a single chip in an SCS region fails, the region may be convertedfrom SCS mode to DCS mode, thus protecting the cache lines in the regionfrom a subsequent chip failure. When a single chip in an SCS regionfails, a DCS base 236-0 may be determined. In addition, a DCS limit238-0 may be determined. Selection of the DCS base and limit will bedescribed in further detail below, but for now, the DCS base and limitdetermine the beginning end end of the SCS region of memory that will beconverted to DCS. For purposes of this description, assume that therehas been a failure of a single chip within memory 215-0 on bus 220-0,and that failure affects the region between the DCS base 236-0 and theDCS limit 238-0 and will be referred to as the first region.

A second region may be selected that is equal in size to the regioncontaining the failure. The second region may also be identified by aDCS base 236-1 and a DCS limit 238-1. In one example, the second regionmay be the identical address range of the first region, except for beingon a different bus 220-1. However, this specific arrangement is notrequired. Examples of additional possibilities are described withrespect to FIGS. 3(a-b). The two sets of DCS base and DCS limitaddresses together define the DCS region.

Conversion pointers 237-0,1 may be set to point to the DCS bases 236-0,1respectively. The conversion pointers may advance from the DCS bases tothe DCS limits, converting cache lines from SCS mode to DCS mode as theyadvance. The process of converting cache lines may be best explainedthough the use of an example. As shown, the conversion pointer 237-0currently points to cache line 262. The corresponding conversion pointer237-1 currently points to cache line 272. The memory controller 210 mayswap one half of each cache line between the first and second region.For example, as shown, cache half line 272-a may be swapped with cachehalf line 262-b using the following procedure. Cache lines 252 and 272are each read from memory and corrected using their corresponding SCSECC, if needed. A DCS ECC is then calculated for each cache line. Theresulting cache lines are then written back to memory. When the cachelines are written back to memory, they are distributed across the firstand second regions. The conversion pointers may then be advanced by onecache line. The end result of this swap being that cache lines 262, 272are now spread across both regions of memory. The process may repeatuntil the conversion pointers point to the DCS conversion limits. Anexample of cache lines that have already been swapped are cache lines260, 261, 270, 271. Cache lines that are waiting to be converted arelines 263, 273.

In the process of swapping the cache lines described above, a new errorcorrection code was computed. However, because the cache lines are nowspread across an increased number of chips, a DCS error correction codemay be used. Once the conversion of a pair of cache lines to DCS mode iscomplete, the cache lines are then able to withstand the failure of anyadditional chip.

While the conversion is in process, any memory access requests to busaddresses that are above the DCS bases, but below the DCS conversionpointers will be processed as access requests to a DCS region. Likewise,any memory access requests to bus addresses that are above theconversion pointers, but below the DCS limits will be processed asaccess requests to a SCS region. A memory request to a bus address thatis pointed to by the conversion pointers, meaning that the address is inthe process of being converted, will stall and will be retried at alater time. By the time the request is retried, the conversion will havelikely completed. If the conversion has not completed, the request willcontinue to stall until such time as the conversion of the bus addressis complete.

The DCS bases and limits were briefly mentioned above. The DCS bases andlimits determine the extent of the regions that will be converted fromSCS mode to DCS mode. As mentioned above, the use of DCS is not withoutpenalty. In particular, DCS regions suffer from reduced bandwidthutilization due to the need to reconfigure the memory controller toaccess locations on different busses. As such it is desirable to limitthe size of the region being converted to DCS to minimize the impacts ofreduced bus utilization.

The techniques described herein select a size of the conversion regionbased on the extent of the memory chip failure. For example, anindividual memory chip may contain individual banks that can failindependently. In the case of a failure of a single bank within a singlechip, the first region may be selected to encompass only that bank. Ifmultiple consecutive banks fail, the first region may include multiplebanks. If the entire chip fails, the first region may include the entirechip. What should be understood is that the first region may be selectedto be as small as possible while still covering all memory locationsthat are impacted by the failure.

FIG. 2 was presented in terms of an example implementation of a memorysystem typically used in computing systems, however it should beunderstood that the techniques described herein are not limited to anyparticular chip configuration, cache line size, or number of busses.

FIGS. 3(a-b) depict additional examples of a memory system with chipfailures. In the description of FIG. 2, it was assumed that conversionof two SCS regions to a DCS region was between memories located on twoseparate busses, and that both SCS regions are at the same relativememory addresses on their bus. However, the techniques described hereinare not limited to such configurations, and may be used with any numberof other configurations. What should be understood is that anyconfiguration of memories, busses, and addresses may be used, as long asan additional chip failure will not affect the ability of the ECC tocorrect for an error. In general terms, any memory configuration inwhich a cache line is re-distributed over a collection of memory chipsto increase the number of detectable and/or correctable memory chiperrors is acceptable.

FIG. 3(a) depicts an example of a double chip spare region 335 that iscontained in memory 315-0 on a single memory bus 320-0. As shown, cachehalf lines (360, 370, 361,371)-a are stored in one range of busaddresses. Cache half lines (360, 370, 361, 371)-b are stored in adifferent range of bus addresses. The configuration shown in FIG. 3(a)is usable with the techniques described herein, so long as any memorychip failure will not affect both halves of the cache line. The reasonthe configuration is valid is that when operating in DCS mode inaccordance with the techniques described, the cache line has alreadyexperienced an error in one chip which affects only one half of thecache line. If an additional chip fails, the failure will also onlyaffect one half of the cache line. Thus, the entire cache line willexperience, at most, two chip failures, which can be corrected by thememory controller 310 when using DCS error correction codes.

FIG. 3(b) depicts another example of a double chip spare region 335 thatis contained in memory 315-0,1 on two memory busses 320-0,1, but theaddresses on those busses are not the same. As shown, cache half lines(380, 390, 381, 391)-a are stored in memory on one bus. Cache half lines(380, 390, 381, 391)-b are stored in memory on a different bus. Asshould be clear, any additional chip failure will affect at most onehalf of a cache line, as the cache line halves stored in chips ondifferent busses are independent of each other. Furthermore, it is notnecessary for the bus address ranges to be the same, as was describedwith respect to FIG. 2. The memory controller 310 may store the addressranges that form the DCS region. Thus, when a memory request isreceived, the proper bus addresses may be accessed to provide thecomplete cache line.

FIGS. 4(a-c) depict an example of a memory system with multiple chipfailures. As mentioned above, the techniques described herein allow forup to two memory chip failures before repairs are needed. However, insome circumstances, it may be possible to reconfigure the DCS regionssuch that additional chip failures may occur and yet still allow thesystem to remain operational.

FIG. 4(a) depicts a system in which a region of memory has beenconverted to operate in DCS mode in accordance to the techniquespresented above. As shown, cache half lines (460, 470, 461, 471)-a arestored in memory on one bus 420-0 and cache half lines (460, 470, 461,471)-b are stored in memory on a different bus 420-1. For purposes ofthis description, assume that there has only been one chip failure sofar, thus the protection provided by DCS is still available. In otherwords, assume that there is only a failure of a single chip on a singlebus and the memory controller 410 can correct the error using the DCSECC.

At some point in time, there may be a second chip failure. In caseswhere the second chip failure occurs on the bus opposite the one thatcontains the first chip failure, the DCS region may be converted back totwo SCS regions, since the ECC for the two SCS regions will besufficient to correct errors caused by the two failed chips. Each of thetwo SCS regions may now be converted into a separate DCS region usinganother error-free SCS region, thus extending the period of DCSprotection.

As shown in FIG. 4(b), assume that there is a failure of a single chipon bus 420-0 and a single chip on bus 420-1. The process of convertingthe region to DCS could then be executed in reverse, resulting in twoSCS regions. As shown, cache half lines are swapped such that bothhalves of a cache line are contained in a single SCS region. As shown,cache lines 460,481 are now contained in a first SCS region, while cachelines 470,471 are contained in a second SCS region, it should beunderstood that each of the SCS regions now has a single chip failure,and as such failure of an additional chip may cause the cache lines tobecome unusable.

However, if another region can be found that does not contain any chiperrors, the process of converting the SCS regions back to DCS regionscan occur again. As shown in FIG. 4(c) a third error free region,containing cache lines 480,481 may be identified. The cache half linesthat made up cache lines 480, 481 may be exchanged with the cache halflines that made up cache lines 470, 471 in accordance with the proceduredescribed above to form a new DCS region. Likewise, a fourth error freeregion, originally containing cache lines 490, 491 may be identified.Again, the procedure described above to swap cache lines between theregion originally containing cache lines 460, 461 and the regioncontaining cache lines 490, 491 may be executed. The result is a newdouble chip spare region, as shown by the dashed lines.

The process of converting a DCS region back into two SCS regions, andthen reconverting back into a DCS region may continue so long as errorfree regions can be found. As such, the amount of time needed beforerepairs are required due to memory failures can be extended until it isno longer possible to find an error tree SCS region, assuming that acache halt line is never affected by more than one chip failure.

FIG. 5 depicts an example flow diagram of recovering from a chipfailure. In block 510 a single chip error in a first region of memorythat is operating in single chip spare mode is detected. Such an errormay be detected by the memory controller through the use of errorcorrection codes. In block 520 a second region of memory equal in sizeto the first region of memory is selected. The second region of memorymay also operate in single chip spare mode. In some cases, the secondregion of memory may be the same range of memory addresses on adifferent bus. In other cases, the second region of memory may be adifferent range of memory addresses, either on the same or a differentbus. In block 530 the first and second regions of memory may beconverted from single chip spare mode to double chip spare mode.

FIG. 6 depicts another example flow diagram of recovering from a chipfailure. In block 605 a single chip error in a first region of memorythat is operating in single chip spare mode is detected. In block 610 afirst double chip spare base is determined. The first double ship sparebase identifies the beginning of the first region of memory. In block615 a first double chip spare limit is determined. The first double chipspare limit identifies the end of the first region of memory.

In block 620 a second region of memory equal in size to the first regionof memory is selected. The second region of memory operates in thesingle chip spare mode. The second region of memory may be identified bya second double chip spare base and a second double chip spare limit. Inblock 625, conversion pointers are set to the first and second doublechip spare bases. In block 630 the two cache lines pointed to by theconversion pointers are read and any errors are corrected using a singlechip spare error correction code. In block 635 a double chip spare errorcorrection code is computed for cache lines in the first and secondregions of memory. For example, the two cache lines may be read and anynecessary correction performed using the SCS ECC. A new DCS ECC may thenbe computed for each cache line. The cache lines and newly computed DCSEGG are stored, as described below.

In block 640 the cache lines and the computed error correction codes arestored into the first and second regions of memory. The cache lines andcomputed error correction codes span the first and second region ofmemory. In block 645 the conversion pointers are advanced to the nextcache lines. In block 650 it is determined if the conversion pointersequal the double chip spare limits. If not, the process returns to block630. Otherwise, the process moves to block 655, in which the conversionis complete.

It should be noted that cache lines may continue to be accessed whilethe conversion is in progress. The cache lines are accessed based on thedouble chip spare bases, limits, and conversion pointers. As explainedabove, cache lines above the conversion pointer are accessed in SCSmode, those below are accessed in DCS mode, and if equal, accessrequests are stalled.

FIG. 7 depicts an example flow diagram of recovering from a chipfailure. In block 710 an error in a portion of a memory operating in asingle chip spare mode is detected. The error may be detected throughthe use of error correcting codes. In block 720 a first region of thememory that fully covers the error may be determined. For example, theerror may be on a single bank of a memory chip, resulting in the size ofthe bank being the size of the region. As another example, the error maybe on the entire chip. In block 730 the first region is converted tooperate in double chip spare mode.

FIG. 8 depicts an example flow diagram of recovering from multiple chipfailures. In block 805 an error is detected in a portion of a memoryoperating in single chip spare mode. In block 810 a first region of thememory that is affected by the error is determined. In block 815 asecond region of memory equal in size to the first region is determined.The second region of memory contains no errors and is operating in thesingle chip spare mode.

In block 820 a double chip spare error correction code is calculated foreach cache line in the first and second regions. For example, each cacheline may be read and any necessary correction performed using the SCSECC. A DCS ECC may then be calculated for each cache line. In block 825each cache line and calculated error correction code is stored in acombined region. The combined region comprises the first and secondregions. In block 830 a second single chip error in the combined regionis detected. The second error occurs in the second region. In block 835the combined region is converted back into the first memory region andthe second memory region. The first and second memory regions operate inthe single chip spare mode.

In block 840 a third region of memory equal in size to the first region,containing no errors, and operating in the single chip spare mode isdetermined. In block 845 a double chip spare error correction code foreach cache line in the first and third regions is calculated. Again,each cache line may be read and any necessary correction performed usingthe SCS ECC. A DCS ECC may then be calculated for each cache line. Inblock 850 each cache line and calculated error correction code is storedin a combined region that comprises the first and third regions.

In block 855 a fourth region of memory equal in size to the secondregion, containing no errors, and operating in the single chip sparemode is determined. In block 860 a double chip spare error correctioncode for each cache line in the second and fourth regions is calculated.Again, each cache line may be read and any necessary correctionperformed using the SCS ECC. A DCS ECC may then be calculated for eachcache line. In block 865 each cache line and calculated error correctioncode is stored in a combined region that comprises the second and fourthregions.

We claim:
 1. A device comprising: a memory controller to: access a firstportion of a memory, the first portion of the memory operating in singlechip spare mode; and access a second portion of the memory, the secondportion of the memory operating in double chip spare mode.
 2. The deviceof claim 1, wherein the memory controller is a memory controller to:detect an error in a first region of the first portion of the memory;and convert the first region containing the error to operate in doublechip spare mode.
 3. The device of claim 1, wherein the memory controlleris a memory controller to: detect an error in a first region of thefirst portion of the memory; select a second region in the first portionof the memory, the second region equal in sire to the first region; andconvert cache lines between the first and second regions to form a thirdregion, the third region operating in double chip spare mode.
 4. Thedevice of claim 3 wherein the first and second regions of the memory areon a single memory bus.
 5. The device of claim 3 wherein the firstregion is on a first memory bus and the second region is on a secondmemory bus.
 6. A method comprising: detecting a single chip error in afirst region of memory, the first region of memory operating in singlechip spare mode; selecting a second region of memory equal in size tothe first region of memory, the second region of memory operating insingle chip spare mode; and converting the first and second region ofmemory from single chip spare mode to double chip spare mode.
 7. Themethod of claim 6 wherein converting the first and second region ofmemory further comprises: computing a double chip spare error correctioncode for cache lines in the first and second regions of memory; andstoring the cache lines and the computed error correction codes into thefirst and second regions of memory, wherein each cache line and computederror correction code spans the first and second region of memory. 8.The method of claim 7 further comprising: determining a first doublechip spare base, the first double chip spare base identifying thebeginning of the first region of memory; determining a first double chipspare limit, the double chip spare limit identifying the end of thefirst region of memory; determining a second double chip spare base, thesecond double chip spare base identifying the beginning of the secondregion of memory; determining a second double chip spare limit, thesecond double chip spare limit identifying the end of the second regionof memory; setting conversion pointers to point to the first and seconddouble chip spare bases; and while the conversion pointers are less thanthe first and second double chip spare limits: read cache lines pointedto by the conversion pointers and correct any errors using single chipspare error correction codes; and advancing the conversion pointers to anext cache line in the first and second regions.
 9. The method of claim8 wherein access of cache lines above the double chip spare bases andbelow the conversion pointers are in double chip spare mode.
 10. Themethod of claim 8 wherein access of cache lines above the conversionpointers and below the double chip spare limits are in single chip sparemode.
 11. The method of claim 8 wherein access of the cache linespointed to by the conversion pointers is rejected and retried at a latertime.
 12. A method comprising: detecting an error in a portion of amemory operating in single chip spare mode; determining a first regionof the memory that is affected by the error; and converting the firstregion to operate in double chip spare mode.
 13. The method of claim 12wherein converting the first region further comprises: determining asecond region of the memory equal in size to the first region, thesecond region containing no errors end operating in the single chipspare mode; calculating a double chip spare error correction code foreach cache line in the first and second regions; storing each cache lineand calculated error correction code in a combined region comprising thefirst and second regions.
 14. The method of claim 13 further comprising:detecting a second single chip error in the combined region, the seconderror occurring in the second region; converting the combined regionback into the first memory region and the second memory region, whereinthe first and second regions operate in the single chip spare mode;determining a third region of the memory equal in size to the firstregion, the third region containing no errors and operating in thesingle chip spare mode; calculating a double chip spare error correctioncode for each cache line in the first and third regions; storing eachcache line and calculated error correction code in a combined regioncomprising the first and third regions.
 15. The method of claim 14further comprising: determining a fourth region of the memory equal insize to the second region, the fourth region containing no errors andoperating in the single chip spare mode; calculating a double chip spareerror correction code for each cache line in the second and fourthregions; storing each cache line and calculated error correction code ina combined region comprising the second and fourth regions.